Final answer:
The nurse should be concerned about potential violence when observing a client clenching fists and pacing, as these behaviors indicate tension that could lead to aggression.
Step-by-step explanation:
The nurse on an inpatient mental health unit should identify the client clenching fists and pacing in the room as an indication of potential violence. This observation is a concerning indicator because these behaviors are consistent with a buildup of tension, which may lead to an aggressive outburst, especially in a client with a history of aggressive behavior. In contrast, behaviors such as maintaining eye contact, calm body language, sitting close and engaging in conversation, or expressing gratitude are less likely to be associated with imminent violence according to research and clinical experience.